Radiosignaling



D ec. 26, 1933. E. H. ARMSTRONG RADIOSIGNALING :s shets-sheet 1 FiledJan'. 24, 1933 Dec. 26, 1933. E. H. ARMSTRONG RADIOS IGNALI NG FiledJan. 24, 1933 3 Sheets-Sheet 2 @AWNV C Tlf mb NQ E j .nw

@www 1 INVENTOR Edwin H. Armstrong. BY

m M Y A TTORNE YS Dec. 26,1933. E H ARMSTRONG 1,941,069

RADIOSIGNALING Filed .I an. 24. 1953 z sheet-seet 3 K M Reacmnc'e /50200 250 Freqaency in Kc.

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Carre/71. /50 Z50 Frequency in Kc.

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N'--- INYENTOR Edwm H. Armsrong.

A TTORNE YS `Patented Dec. 26, 1933 -UNITED STATES l 1,941,069RADIOSIGNALING Edwin H. Armstrong, New York, N. Y.

Application January 24,

2 Claims.

This invention relates to a method of increasing the distance oftransmission which may be covered in radio signaling with very shortWaves.

It is well known that waves of the order of ten meters or lower arelimited in the distance of transmission by tube noise alone as theamount of static in that part of the spectrum is negligible.

That is, the electrical disturbances which are created in the rst tubein a chain of vacuum tube amplifiers, when amplified by the chain andsupplied to the detector are great enough to mask the effect of thesignal when it falls below a certain level.

There is described herein a method of overcoming this type ofdisturbance by the u se of frequency modulation in a particular way, sothat it may be very greatly decreased with consequent increase in thedistance over which communication may be maintained. In order to fullyunderstand the nature of this invention, it is necessary to consider thecharacteristic of the disturbance and the present state of the art ofreceiving weak signals.

The nature of the disturbance, which is due mainly to the irregularitiesof the electron emission from the filaments of the vacuum tubes, is thatof a spectrum, containing all frequencies, and as is well known itmanifests itself in the telephone by a high pitched hiss, thefrequencies composing which run from some low value to above audibility.The combination of all the irregularities of emission produces aspectrum of radio frequency currents which consists of irregularvariations in amplitude and also, as I have found, in frequency, so thatthe hiss is heard in a frequency modulation receiver as well as in theordinary type of receiver for amplitude modulated waves. This occurseven when the amplitude modulations of the disturbing currents areeliminated by current limiting or by some equivalent process.

It is the practice in designing amplitude modulated receivers to designthe width of the selective system to be equal to twice the frequency ofthe modulation to be received and in voice transmission or thetransmission of music this runs between 10,000 cycles and 15,000 cycles.

In frequency modulation, while there is no practice, the experimentationhas proceeded along the same lines, the Width in this case beingsomewhat greater than in the amplitude modulated case in order to allowfor the deviation in frequency.

That is, the ordinary width dependent on the modulation frequency isincreased by an amount 1933. Serial No. 653,237

(Cl. Z-2) dependent onk the frequency. deviation. These band widths havealways been kept down to as low a value as possible because the amountof 1 static which is received is proportional to the Width of the bandand hence after providing for the signal there is no advantage in goingfurther. This applies both to amplitude and frequency modulated waves.

Where, however, there is no static and the sole disturbance is tubenoise a different situation 05 occurs. It is still true in the case ofamplitude modulation that the band width should be approximately twicethe frequency of modulation and no greater. It is not true in the caseof frequency modulation.

I have discovered that by imparting a greater swing to the frequency ofthe transmitted wave than can exist in the disturbancesdue to tubeirregularities and providing means for selecting these large swings offrequency which are at the same time substantially not responsive to thelesser swings due to the tube disturbances Aor to the variations inamplitude due to these disturbances, that a very great improvement intransmission can be produced.

Referring now to the figures which form a part of this specification,Fig. l illustrates the general arrangement of the transmitter, Fig. 2the arrangement of the receiver and Figs. 3, 4 and 5 the characteristicsof reception of the arrangement of Fig. 2.

In Fig. 1 is shown a modulating system similar in principle to thatdescribed in my application filed of even date herewith. Here 1represents a constant frequency oscillator, 2 an amplifier of the outputof this oscillator with a resistance 3 in its plate circuit which issmall in comparison with the impedance of the tube. 4 and 5 are likewiseamplifiers of the currentproduced by the master oscillator 1. 6 is atransformer for dif'- 95 ferentially modulating the plate voltages ofthe tubes 4 and 5 by the signaling current which is applied to theprimary of the transformer through the amplifying system 26, 29, 31. 7and 8 are condensers shunting the two halves of the secondary of thetransformer 6, and 9 and 10 are inductances whose impedance for thefrequency of the oscillator is small compared to the impedance of thetubes 4 and 5. 11 is a small induct- 105 ance whose natu-ral frequencyis high compared to the frequency of the oscillator. It is coupleddifferentially to the coils 9 and 10. /12 is an vampliiiei" foramplifying the outputs of the tubes 4 and 5. Its plate is connected asshown to an 110 adjustable point in the resistance of the plate circuitof the amplifier 2. The combined outputs of the tubes 2 and 12 aresupplied to an amplier 15, 16 whose output passes through a currentlimiter 17, lter 18 and to a frequency multiplier 19 of many stages,power amplifier 20 and antenna 22. 27, 28 represents a correction systemfor impressing on the input, of the amplifier 29 a voltage inverselyproportional to the frequency of modulation which is applied at 25.

Referring now to Fig. 2 which represents a receiving system, 35, 36represent the antenna system. 37 represents an amplifier for thefrequency of the received wave. 38 represents a rectifier and 39 anoscillator for heterodyning the output of the amplifier 37 to a lowervalue which is amplified in the amplifier 40. 4l, 42 represents arectifier and oscillator for heterodyning the output of the amplifier 40down to a still lower intermediate frequency and 43 represents anamplier for this second intermediate frequency. 44 represents a currentlimiter supf plied by the output of the amplifier 43. 45 is a filterthrough which the output of the current limiter 44 is passed. 46 is asecond amplifier similar to the amplifier 43 and 47 is a current limiterfor limiting the output of this amplifier. 48 is a filter for the outputof the second current limiter 47. 49 is an amplifier for amplifying thelimited and filtered output of the amplifier 46. The output of theamplifier 49 is delivered to a selective system consisting of two branchcircuits 50, 52, 54 and 51, 53, 55 which are connected respectively tothe inputs of the two detectors 56, 57. The plate circuits of thesedetectors contain transformers 58, 59 which are connected so as torespond cumulatively for frequency variations, but differentially foramplitude variations. 66 is a meter connected across the bridge formedby the resistances 62, 63 for the purpose of indicating the balancepoint. 60, 61 and 64, 65 are the usual by-pass condensers. 67 is a lterfor the purpose of excluding frequencies above the range of thesignaling frequencies and 68 is the telephone receiver.

The general characteristics of detection are illustrated in Figs. 3 and4. Fig. 3 illustrates the reactance characteristics of the capacityinductance combinations 52, 54 and 53. 55. Characteristic values aretaken for a swing of 100,000 cycles between the limits of 150,000 and250,000 cycles respectively. In this ligure M represents the reactancecharacteristic of 52, 54 which is made non-reactive for 150,000 cyclesand N the reactance characteristic of 53, whichI is made non-reactivefor 250,000 cycles.

Fig. 4 illustrates the rectified or plate currents through theresistances 62 and 63 as shown by M and N, and O represents the currentthrough the indicating instrument 66 in the balanced arm of the bridge.The output of the two transformers 58 and 59 are proportional to thecurrent changes indicated by O. Fig. 5 illustrates the action of thesystem with respect to disturbances and will be referred to in detaillater.

In principle the operation of the transmitter is similar to thatdescribed in my application previously referred to except that in thepresent arrangement the number of frequency multipliers is increased toan extent which gives a swing many times greater than the audiblefrequency range.

In the receiver the same general method of operation regarding thesignal occurs as is described in my pending application .Serial NO,192,-

320, filed May 18, 1927 for Radio telephone signaling. That is, themethod of translating the radio frequency currents of variable frequencyinto currents of audible frequency which is shown in that specificationis employed. However, because of the design of the selective systemwhich is used in the present system the operation of the receiver isquite different from that described in the preceding application.

Because of the extremely complicated detecting action occurring in thissystem which results in the cumulative detection of signal and thedifferential detection of the disturbances originating in the tubes andthe fact that some phases of the operation are obscure even to thoseskilled in the art it is necessary to deal with an explanation of thematter in great detail.

As already stated it is the standard practice in receiving amplitudemodulated waves to design the receiving apparatus to have a frequencyband width of admittance slightly greater than twice the frequency ofmodulation which it is desired to receive. This practice is used becauseit has been found that the amount of energy received from atmosphericdisturbances varied directly as the width of the frequency band passedby the receiver and hence there was a disadvantage in making the bandany wider than that necessary to pass the signal.

In the use of frequency modulation, while there is no practice,experimental work on ordinary commercial frequencies where there areatmospheric disturbances has shown the same general rule to beapplicable except that the receiver must be designed to pass a somewhatgreater band width than the amplitude modulated receiver,

(assuming the same frequency of modulation). That is, the band widthmust be twice the frequency of modulation, plus the frequency swing ordeviation.

On wave lengths belowten meters, however, atmospheric disturbancespractically cease to exist, or in any event become of negligible amountcompared to tube noise, and the limit of reception is then determined bytube noise or the disturbances which arise usually in the first tube inthe receiving system.

The interference manifests itself as a steady hiss in the telephones orspeaker and it is quite disturbing even when its amplitude is smallcompared with the amplitude of the Signal. Electrically it ispractically a continuous spectrum. In this it differs from static inthat static is an extremely irregular spectrum in which, because of itsdiscontinuous character, the peaks may be commensurate with or greaterthan the signal before serious disturbance occurs.

In order to understand the operation of this system with respect to acontinuous spectrum of substantially constant amplitude it is necessaryto analyze carefully what occurs in an ordinary receiver for amplitudemodulated waves which is admitting a broad band of frequencies (severaltimes that necessary to pass the frequencyv of modulation) at a wavelength where tube disturbances are the predominating factor. Suppose forexample the width of the receiver band is 100,000 cycles. Now when nosignal carrier is being received the energy that is received thruoutthis band is uniformly rectified by the detector to produce a responsewhich manifests itself as a continuous hissing tone. All parts of thespectrum within the 100,000 cycle band contribute to the response whichis heard in the telephone receiver. Now suppose an unmodulated carrieris lilir received which is of the same order of magnitude or greaterthan the .tube disturbances. Under these circumstances the actionchanges and -two things occur. One is that because of the presence ofthe carrier an increase in efficiency of detection occurs. The natureofthis phenomena is well understood in the art. of it may be found inthe Proceedings of the Institute of Radio Engineers for April 1917,where it was rst described by me. The other is that the energy which iscapable of producing an audible response is narrowed down from a band of100,000 cycles in width and confined to the band which lies within theaudible range on either side of the carrier. As a practical matter thismay be taken to be 10,000 cycles above and 10,000 cycles below thecarrier, so that the effect of the receipt of the unmodulated carrier isto narrow down the band from which the tube disturbances will produce anaudible response. While the tube disturbances in the rest of the bandare passed by the receiver as before and are rectified by the detectorthe currents resulting from this rectification have a frequency aboveaudibility and are therefore not audible in the telephone receiver.Although the energy which is now able to produce an audible response islimited to that within a band only onefifth as wide as before, it doesnot follow that the response in the telephones is correspondinglyreduced. In general it is somewhat increased, since the presence of thesignal carrier produces an increase in the eiciency of rectificationwhich more than makes up for the narrowing of the band, althoughif thevoltage applied to the detector is sufficient and the detector issubstantially linear there is relatively little difference in theaudible response.

In the above considerations it has been assumed that there is no currentlimiting. With current limiting an additional effect occurs. Thepresence of the carrier raises the amplitude level beyond the limitingvalue so that a large part of the fluctuations in amplitude created bytube disturbances are wiped out, leaving only the variations infrequency created by this type of disturbance.

Now with this conception in mind examine the action of a balancedamplitude receiver for frequency modulated waveshaving thecharacteristics ilustrated in Fig. 5. This characteristic is the same asthat illustrated in Fig. 4 but it is shown in Fig. 5 in greater detail.Assume for the moment that the signal is not being modulated and that asteady carrier of 200,000 cycles is being transm1tted.

The only components of this spectrum from 150,000 cycles to 250,000cycles which can at any one time by reason of amplitude modulationproduce a really audible noise with the carrier in either of the twodetectors are those components lying within, say, 10,000 cycles oneither side of the signaling frequency, wherever that may happen to be.If we assume for the moment that the signal is not being modulated and asteady frequency of 200,000 cycles is being transmitted, then only thedisturbances between 190,000 cycles and 210,000 cycles can produce anycurrent which is of a really audible character.

An examination of Fig. 5 wil show that the reactance at 200,000 cycleson curve AB is equal to the reactance at the same frequency on curve CD.Hence the voltages impressed on each detector will be equal and therectified currents will also be equal. As previously shown the only partof the band which is capable of producing an A technicalexplanationaudible response-due to amplitude variations are thosefrequencieslying between 190,000 and 210,-

000 cycles per second. It will be observed that between these rangesthere is relatively little dif-` ference between the reactances oneither side and thattherefore insofar as the amplitude variations Awhichremain after limiting are concerned the voltages impressed on the twodetectors are substantially balanced. The diiference between thevoltages OM and ON represents approximately the response which will beheard in the telephones due to amplitude variations, and as these can bemade substantially equal those amplitude variations which pass thecurrent limiter are cancelled out.

As regards frequency variations produced by currents lying within therange capable of producing audible response, that is, from 190 to 210kilocycles the action is cumulative. The response cn the siderepresented by the curve AB is equal to OP-OQ or PQ and on the siderepresented by CD the response is equal to OS-OR or SR. Since the twodetector outputs are connected cumulatively for frequency variations thetotal response is the sum of the two or PQ-l-SR..

As regards the signal, however, by reason of its frequency swing overthe range from 150 kilocycles to 250 kilocycles the voltages impressedon the detectors Vary from zero to OM' on one side and from zero to ONon the other side. The total response is, therefore, proportional to thesum of OM' and ON and the improvement in sgnal to hiss ratio isapparent.

While the above explanation shows the fundamental reason for theimprovement obtained by this method its quantitative result must not betaken too literally since it has been assumed` that the range 190 to 210is the limit of the band which represents the sole source of the audiblehiss. This is not strictly true since there are certain second ordereffects. But While there is no strict line which can bedrawn, theextent'of the swings of .frequency due to tube disturbances which canproduce audible disturbing response does not apear to be much greaterthan the value of the example given. It follows therefore that byincreasing the variation of frequency of the transmitted wave to valuesgreater than those indicated in the figures a still greater improvementin signal to noise ratio can be obtained. I find this to be so and alsothat it is readily practicable on the shorter wave lengths.

As a consequence of this method of signaling it now becomes possible toopen up ranges of wave lengths and to operate over distances which werequite impossible with the methods previously known.

Based on measurements made with the system illustrated in Figs. 1 and 2with a frequency swing of 100,000 cycles and a transmission frequency of50,000,000 cycles (6 meters) the disturbances were reduced by thismethod to less than 1% of the energy of the disturbances in an ordinaryamplitude modulated transmitter of equal power. For the purpose ofcompleting the disclosure and to enable those skilled in the art topractice the invention the following added description of the systems ofFigs. 1 and 2 used in these tests are here given. The initial frequencyor the frequency of the master oscillator in the transmitter was of theorder of 50,000 cycles. Ten stages of frequency doubling was employed.The tuned circuits in these stages were suitably broadened by theintroduction of resistance to accommodate the wide variation offrequency. On account of ne' we."

4l Laarne@ ,4

this the selectivity of the doubler circuits for other frequencies isreduced so that push-pull circuits to eliminates the fundamental of thepreceding stage are employed. In the phase shifting system coils 9 and10 have an inductance of 5.4 mil henrys and coil 11 an inductance of 9.6mil henrys. Condensers 7 and 8 have a capacity of .002 mfds. In thecorrection system the value of the resistance 27 is 150,000 ohms and thevalue of the capacity 28 is .2 mfds.

In the receiving system the amplier represented byv 37 consisted ofthree stages of tuned circuit coupled amplification at 50,000,000cycles. This was heterodyned down by the rectifier, oscillatorcombination 38, 39 to about 6,000,000 cycles and amplied by the fourstage transformer coupled amplier 40. The output of this amplifier washeterodyned down by the rectifier, oscillator combination 41, 42 to200,000 cycles. This current was then amplified by a ve stage resistancecoupled amplifier arranged to be flat from 150,000 cycles to 250,000cycles by the proper choice of plate resistances, grid condensers andgrid leak resistances. The output of this amplifier is supplied to acurrent limiter consisting of a screen grid tube operated at less thannormal plate and screen grid voltages. This makes an eective currentlimiter provided suiiicient excitation is applied to the grid and forthat reason the large number of stages in the 200,000 cycle amplifier isused. I have discovered, however,that a repetition of the process ofamplification and current limiting is Very advantageous and therefore asecond resistance coupled amplier 46 and a second current limiter 47similar respectively to the amplifier 43 and current limiter 44 is used.The reason for this is that the action of a current limiter is such thatWhile it eliminates amplitude Variations at the frequency of the carrierit does not eliminate the variations in amplitude produced by theinteraction of other frequencies Within the band of the receiver. Whilethese are second order effects they are appreciable and are best dealtwith by the use of a second or third current limiting system. Theamplier 49 consists of a couple of stages of resistance coupledamplification for the purpose of raising the voltage applied to thegrids of detectors 56, 57 (which are biased to cutoff) to a sufficientlevel to secure straight line rectification. The resistances 50, 51 areof the order of 15,000 to 20,000 ohms and the inductance, capacitycombinations 52, 54 and 53, 55 are suitably chosen, one to benon-reactive slightly below 150,000 cycles and the other to benon-reactive slightly above 250,000 cycles, and to have, respectively,the same arithmetic value of reactance at 200,000 cycles.

I have described what I believe to be the best embodiment of myinvention. I do not Wish, however, to be confined to the embodimentshown, but what I desire to cover by Letters Patent is set forth in theappended claims.

I claim:

1. The method of eliminating in radio signaling disturbances having thenature of a spectrum, which consists in producing a variation infrequency of the wave to be transmitted substantially greater in extentthan the frequency range of good audibility, transmitting such wave,receiving the wave and amplifying the received currents, substantiallyeliminating amplitude 'variations so as to minimize the noise caused bythe amplitude variations due to the disturbances, translating thefrequency variations of the received signal into amplitude variations bya selective system which is fully responsive to the Wide variations ofthe signal, but substantially not responsive to the lesser variations infrequency of the spectrum of the disturbances to be eliminated.

2. A system for eliminating in radio signaling disturbances having thenature of a spectrum, comprising means at the transmitter for producinga variation in frequency of the wave to be transmitted, substantiallygreater in extent than the frequency range of good audibility, means atthe receiver for amplifying the received currents, current limitingmeans for substantially eliminating amplitude variations, and a detectorsystem for translating the frequency variations of the received signalinto amplitude variations, said system being fully responsive to thewide variations of the signal but substantially not responsive to thelesser variations in frequency of the spectrum of the disturbances to beeliminated nor to variations in amplitude of said disturbances.

EDWIN H. ARMSTRONG.

